This invention relates to motion picture projectors and more particularly to an improved electronic transport system for intermittently advancing film through a light gate in a motion picture projector.
Film projectors used in commercial motion picture theaters typically comprise a high intensity light source, film supply and take-up reels and a drive system therefor, and a projection head including a light gate through which the film passes, a transport for intermittently advancing the film through the light gate one frame at a time, a shutter and a lense system. The transport in the projection head often comprises a single motor which drives both the shutter and a Geneva mechanism which periodically rotates a sprocket through a predetermined increment to advance the film through the gate. The Geneva mechanism is synchronized with the shutter through a gear drive such that the sprocket is rotated by an amount necessary to advance the film through the gate by one frame each time the shutter interupts the projected picture. While a picture is projected, the Geneva mechanism inhibits movement of the sprocket to prevent any movement of the film in the gate.
Geneva mechanisms of a precision necessary for commercial motion picture projectors are quite expensive because of tight tolerance requirements. The mechanism must accurately advance the film in the gate to maintain a constant film alignment in the gate for each successive frame. If there is even a slight variation in the alignment of successive frames in the light gate, the projected image will jump or jitter. When the image is projected on a large screen, such jitter is extremely annoying to the viewer, and particulary to viewers seated nearest the screen.
My U.S. Pat. No. 4,022,525 is directed to an electronically controlled servomotor for intermittently advancing motion picture films through a light gate in a projection head and for precisely indexing each successive frame with the light gate. The film is incrementally advanced through the projection head by a servomotor system which is controlled in synchronism with the position of a shutter. The servomotor is driven to advance the film through the light gate by one frame when the shutter is in a position blocking the projected picture. The servomotor drive has several advantages over a Geneva mechanism. For example, the servomotor may be operated on an intermittent basis for advancing the film through the light gate one frame at a time or it may be driven on a continuous basis for rapidly moving the film forward or backward through the light gate, for example, to rapidly rewind the film back through the projection to automate the projector system. This is not possible with a transport using a Geneva mechanism since the Geneva mechanism moves the film on an intermittent basis both forward and backward and is limited in its maximum operating speed.
The light gate through which the film is intermittently advanced is positioned between a high intensity light source and a projection lense system. One purpose of the light gate is to hold a film frame either flat or with a predetermined curvature as the frame is being projected onto a screen. The light gate may be designed to impart a small curvature to the film frame in order to provide a better focus when the frame is projected on to a curved screen. In order to hold the film flat or with a predetermined curvature, edges of the film are normally clamped between spring biased guides. However, the spring biased guides may eventually cause the edges of the film to wear, thereby limiting the life of the film. For larger format films, such as 70 millimeter films, clamping the edges of the film in the light gate may be insufficient to provide a desired flatness or curvature to the film frame during projection. This is further complicated by the fact that heat from the light source heats the film in a non-uniform manner and tends to warp the film during projection of a picture. In one commercially available large format projector, the film is pressed or held against a glass plate in the light gate during projection of a picture. The glass plate is shaped to provide a desired flatness or curvature to the film frame during projection. When the film is to be advanced, a compressed air jet forms a loop into the film and this loop is then rolled down one frame past the glass plate in the light gate. However, this system has proved extremely expensive to construct.